1. Field
One or more aspects of embodiments according to the present invention relate to analyzing received radar pulses and in particular to de-interleaving interleaved radar pulses received from multiple sources.
2. Description of Related Art
When a military asset such as an aircraft is simultaneously illuminated by multiple radar systems, it may be useful to sort the received radar pulses into groups, or clusters, according to their source. An asset may, for example, be illuminated by several different radars from different directions, each having a different combination of amplitude, carrier frequency, pulse width, and modulation type. The received radar pulses may be detected and analyzed by a radar analysis system such as a radar warning receiver (RWR), which may then identify each source as being, for example, ship-board radar, aircraft radar, or missile radar.
Radar pulses from multiple sources may be interleaved in time when they arrive at the radar analysis system. As a first step in analyzing the received pulses it may be desirable to de-interleave them, i.e., to form multiple individual pulse trains, each corresponding to one of the radar sources. This may be particularly important for determining certain characteristics of each source: in a stream of pulses from several sources, for example, each operating at a different pulse repetition rate (PRR), it is difficult to infer the PRR of any one source without first separating, i.e., de-interleaving, the stream into separate pulse trains, one pulse train for each source. Once the stream is de-interleaved the PRR of each source is, in the ideal case, simply the rate at which pulses occur in the corresponding pulse train. Even if some pulses are lost in processing, the PRR of a given source may be estimated from the remaining pulses, for example by inserting pulses as needed to produce a regular sequence of pulses. Other attributes such as the carrier frequency may be determined from a single pulse, but it may be possible to determine it more accurately from a series of pulses originating from the same source.
Prior art systems may perform de-interleaving by grouping received pulses into “clusters” of similar pulses. Pulses that are, according to some measure of similarity, sufficiently similar, are deemed to have originated from a single source, and pulses that are dissimilar are deemed to have originated from different sources. Various attributes, or parameters, may be compared when assessing the degree of dissimilarity, or “distance,” between pulses, including frequency, the direction from which the pulses arrived or angle of arrival (AOA), pulse width, and amplitude. A weighted measure of distance may be used. In such a measure a weight may be defined for each parameter and differences in any parameter multiplied by the corresponding weight.
Selecting the weights for a weighted distance generally involves making a compromise between the respective likelihoods of two types of error. If a weight is made too large, pulses originating from the same source may be incorrectly classified as originating from different sources; if the weight is made too small, pulses originating from different sources may incorrectly be classified as originating from the same source. If the measurement error in a parameter is large, it may be preferable to use a small weight, so that variations in the measured value due to measurement error do not cause pulses, which in fact are quite similar, to be classified as originating from different sources. Thus the optimum weights may vary, depending on operating conditions. At high temperature, for example, amplifier noise in the receiver may cause the measurement error for some parameters to increase, with the result that smaller weights may be preferred at higher temperatures.
In prior art de-interleavers, the weights may be fixed prior to operation, and they may be independent of the pulse parameters. Weights chosen for low temperature operation may then be too large at high temperatures, and, conversely, weights chosen for high temperatures may be too small for good performance at low temperatures. Similarly the optimum weights may also depend on the parameter values. The measurement error in pulse width may be greater, for example, for short pulses than for long pulses, so that weights chosen for small pulse width may be too small when used for long pulses, and weights chosen for long pulses may be too large when used for short pulses. Thus there is a need for a de-interleaving system capable of performing well over a range of operating conditions and pulse parameters.